Abstract: A cut-off filter is based on the use of light at non-normal angles of incidence and on the use of coating materials with a large dispersion of the optical constants. The filter has a pair of optical media respectively defining an inlet port and an outlet port and having an inclined interface such that a light beam passing between the inlet and outlet ports strikes the interface at a non-normal angle of incidence. An active core is sandwiched between the optical media at the interface. In one embodiment, the active core has a refractive index which has high dispersion in the vicinity of the predetermined cut-off wavelength and an extinction coefficient which is high on the cut-off side and low on the transmission side. The angle of incidence &agr; is selected such that total internal reflection occurs at the predetermined wavelength &lgr; to bring about a sharp cut-off in the transmittance of the filter at the predetermined wavelength.